1. Field of the Invention
The present invention relates to sewing technology, and more particularly it relates to a machine for producing triple-thread chain stitch for sewing webs over an edge.
2. Description of the Prior Art
There is known a triple-thread chain stitch, wherein the first thread is positioned at one side of the webs being sewn and forms a series of straight loops parallel with the edge of the webs, and interweaves with the second thread forming a similar series of loops at the other side of the webs, while the third thread forms straight overcasting or whip loops, each connecting two adjoining loops of one series and having its legs received in one loop of the other series (cf. F. I. Chistyakov, A. M. Nikolaenko, "Sewing Machines", Moscow, MASHINOSTROJENIE Publishers, 1976, p. 380). The knots formed by the interweaving of the first and second threads are disposed to one side of the webs, while the knots produced by the interweaving of the overcasting loops with the loops of the first and second threads are either disposed to the opposite sides of the webs or so offset that a succession of these loops is situated outside the edges, at the borderline between the webs being sewn. The legs of the straight overcasting loops are running either parallel or at a small inclination toward each other.
A known method of producing such a triple-chain stitch, performed by a sewing needle carrying the needle thread and by the first and second loppers carrying, respectively, the first and second looper threads, includes the steps of:
piercing the webs with the needle, having pulled the needle thread in advance through a loop of the first looper thread;
pulling the second looper thread through the loop of the needle thread, drawn through the webs;
pulling the first looper thread through the loop of the second looper thread; and
casting the loops thus produced in succession off the loopers and tightening the stitch.
However, experience of employing this stitch for sewing fishing nets has revealed insufficient strength of the produced seams, manifesting itself in not unfrequent cases of mechanical damage of the fishing gear. The destructive effect of such damage is aggravated by the fact, when either one of the threads becomes broken, the chain stitch where all the three series of loops are straight readily loosens in the direction opposite to that of its formation.
There is further known a triple-thread chain stitch for sewing up webs over their edges, wherein the first thread forms a series of straight loops at one side of the webs being sewn, the second thread forms a series of straight loops driven through the loops of the first thread and through the webs being sewn to their other side, and the third thread forms a series of turned loops with crossing legs which project through the loops of the second thread and encompass the loops of the first thread (cf. SU Inventors' Certificate No. 820292; Int.Cl..sup.3 D 05 B 1/08, published July 30, 1983).
There is further known a method of producing such a triple-thread chain stitch for sewing webs, e.g. netting webs, over their edge, performed by successively driving the needle thread through a loop of the looper needle thread and through the webs, and subsequently driving a turned loop of the looper hook thread, through the loop of the needle thread. This yields a triple-thread stitch including two series of straight loops and one series of turned loops with crossing legs.
This method is performed by a machine comprising a framework, means for guiding the webs into the sewing zone, means for retaining the sewn up edges of the webs, a device for advancing the webs and the main shaft operatively connected with mechanisms for driving the needle mounted on a needle bar, the looper needle mounted on a rod and the hooper hook mounted in a cantilever fashion on a telescopic rotatable bar.
The telescopic rotatable bar having the looper hook mounted thereon in a cantilever fashion extends above the horizontal needle bar, running parallel therewith and belonging to one and the same plane. The looper hook in its extreme position overlies the needle at the outer side of the means for retaining the edges of the sewn webs, while the needle has its eye for outlet of the sewing needle thread facing upwardly and is freely received in the gap between the means guiding the webs into the sewing zone and means for retaining the sewn up edges of the webs.
The tip of the looper hook which is in the form of a straight hollow stem is upwardly pointed. The guiding cam of the mechanism driving the looper hook is in the form of a cylindrical disk with a valley smoothly conjugating with the cylindrical surface of the disk.
Owing to the turned loops, the third thread of a chain stitch of this type is somewhat better linked with the second thread. If the third thread is broken, its turned loops tend to tighten on the loops of the third thread, thus restraining to a certain degree the loosening of the chain stitch. This amounts to definite advantages of a stitch of this type over the first-mentioned stitch, and therefore netting webs joined by this stitch have become broadly used in the production of industrial fishing gear.
However, in this stitch, too, the knots connecting the first thread (needle thread) with the second and third threads (looper needle and looper hook threads, respectively) are formed exclusively by the interweaving of straight loops. Therefore, when either the first thread or the second one becomes broken, the chain stitch loosens as readily as in the first-described case. Furthermore, the turned loops of the third thread, interconnecting the pairs of adjacent straight loops of the second thread, are uncommonly large and would not be made smaller on account of the very structure of the seam, whereby the turned loops are all but similar to straight loops, and, consequently, inadequately effective because when the third thread becomes broken, they tighten but in some cases, and in most cases become spread giving rise to self-loosening of the seam. Thus, to prevent self-loosening of a triple-thread chain stitch of this structure used for sewing up netting webs intended for industrial fishing gear, a seam thus formed is stitched at certain intervals with an additional second seam produced manually in intermittent stitches superimposed from above upon the main seam, which steps up the input of labour into the manufacture of the fishing gear.